Chromium is a strategic metal in the United States because of the nearly 100% import dependence and wide variety of important uses. Among the most critical uses for chromium is providing high temperature and oxidation resistance in both cobalt and nickel-based superalloys. In order to be utilized in this application, the chromium must be of high purity. Other common uses for the metal, for example, in stainless steel have less stringent purity requirements.
Annually, many millions of pounds of superalloy material are removed from service and are downgraded as scrap. Much of the scrap superalloy is exported from the United States or used domestically for steel production. The value of the scrap is many times less than the value of the new alloy and in some cases is less than the value of the constituent metals. Therefore, this practice represents a larger dollar loss as well as a loss of strategic metals from the United States economy.
The recycle of superally materials, other than direct remelting has been economically unfeasible up to this time. In part, this has been due to the lack of technology for dissolution of large amounts of material at reasonable rates. In addition, since most of the chemistry for the production of chromium metal is based on chromite ore as a starting material, separation methods to remove elements found in used superalloys but which do not naturally occur in chromite have not been fully developed.
The following U.S. Patents relate to processing of chromium-bearing material:
U.S. Pat. No. 4,668,483 relates to recovery of chromium from material containing cobalt, molybdenum, and tungsten by roasting the material with an alkali metal compound to form reacted material containing alkali salts of chromium, molybdenum, and tungsten, water leaching the reacted material to form a solution of the salts and a solid containing the cobalt. The Cr-Mo-W solution is processed to recover Cr, Mo, and W by precipitation techniques. Chromium is precipitated upon addition of ammonium sulfate to the solution on the acid side.
U.S. Pat. No. 4,608,235 relates to a method for processing of stellite material emphasizing the recovery of cobalt therefrom. The material is fused with sodium hydroxide to form water soluble salts of chromium and tungsten and hydroxides of cobalt and nickel. The fused material is leached with water to solubilize the chromium and tungsten salts. The cobalt hydroxide remains as a solid.
U.S. Pat. No. 4,608,084 relates to digesting the chromium bearing material in hydrochloric acid to form a solution containing the major portion of the cobalt and chromium which is then further processed to precipitate cobalt as cobalt oxalate.